The present invention relates to an epitaxial wafer for fabricating a high-intensity infrared light-emitting device which is employed in an optical communications and spatial transmission apparatus using infrared radiation. The invention also relates to an infrared light-emitting device fabricated from the epitaxial wafer and to an optical communications and spatial transmission apparatus employing the device.
Light-emitting devices (hereinafter referred to as LEDs) employing GaAlAs compound semiconductors have been widely used in a light source in a wavelength range from infrared to visible red light. Although an infrared LED is employed in optical communications and spatial transmission, there has been increasing demand for a high-intensity infrared LED of increased capacity for transmitting data and increased transmission distance.
As has conventionally been known, a GaAlAs LED having a double-hetero structure (hereinafter DH structure) exhibits emitted-light intensity higher than that of a GaAlAs LED having a single-hetero structure, and emitted-light intensity is enhanced by means of removing a substrate.
In fabrication of an LED employing a substrate-removed-type structure (hereinafter referred to as a DDH structure), a typical DH structure; i.e., only three layers consisting of a p-type cladding layer, an active layer, and an n-type cladding layer, is epitaxially grown and then a substrate is removed, to thereby reduce the thickness of a produced epitaxial wafer. Such an epitaxial wafer is difficult to handle during processing into a device. In addition, the distance from a bottom surface of the device to the junction decreases, and a paste for bonding the device to a conductor migrates through a side face of the device, to thereby disadvantageously short-circuit the pn junction. In order to avoid this problem, a fourth epitaxial layer is added to the DH structure so as to ensure the overall thickness of the substrate-removed and finished epitaxial wafer and the distance from a bottom surface of the device to the junction. This constitution is standard for a DDH structure. The fourth epitaxial layer is designed to have a band gap wider than that of an active layer, so as not to absorb emitted light from the active layer.
The fourth epitaxial layer is advantageously formed as an n-type layer on the side of an n-type cladding layer, in consideration of suppression of overall electric resistance of a device, since in a GaAlAs system electron mobility is 10 or more times hole mobility. Thus, an n-type layer has an electric resistance lower than that of a p-type layer when carrier concentration and Al compositional proportion in two layers are identical. When an n-type layer is formed on the side of an n-type cladding layer so as to dispose a p-type cladding layer as an LED surface, two arrangements are possible. In one case, as shown in FIG. 2, an n-type GaAs substrate is employed, and on the substrate, a second n-type layer 6, an n-type cladding layer 5, a p-type active layer 4, and a p-type cladding layer 3 are sequentially formed. In the other case, as shown in FIG. 3, a p-type GaAs substrate is employed, and on the substrate, a p-type cladding layer 3, a p-type active layer 4, an n-type cladding layer 5, and a second n-type cladding layer 6 are sequentially formed.
During liquid-phase epitaxy, Te is employed as an n-type dopant. The segregation coefficient of Te increases with decreasing temperature. Therefore, when epitaxial growth is initiated from the n-type substrate 1 as shown in FIG. 2, carrier concentration at the interface between the active layer 4 and the n-type cladding layer 5 increases as compared with carrier concentration obtained through epitaxial growth initiated from the p-type substrate 1 as shown in FIG. 3. Thus, in the constitution shown in FIG. 2, reverse withstand voltage decreases and non-radiative recombination centers are formed due to deterioration in crystallinity at the interface caused by high-concentration Te, thereby lowering intensity of emitted light from a device fabricated by use of the epitaxial wafer.
When Zn, a typical dopant, is employed as a dopant in the p-type cladding layer 3 during epitaxial growth initiated from the p-type substrate 1 as shown in FIG. 3, the active layer 4 must be grown at high temperature, and Zn incorporated in the p-type cladding layer 3 penetrates the active layer 4, thereby diffusing to the n-type cladding layer 5. As a result, a pn junction is shifted to the n-type cladding layer 5, to thereby generate an intermediate layer. The generation of the intermediate layer lowers intensity of emitted light of an LED and elevates forward voltage (VF), to fail to attain required device characteristics.
In order to solve the aforementioned problems, an object of the present invention is to provide an epitaxial wafer for fabricating an infrared LED which has a DDH structure and exhibits high emitted-light intensity, low VF, and excellent device characteristics. Another object is to provide an infrared LED fabricated from the epitaxial wafer.
The present inventors have carried out earnest studies on reduction of diffusion of dopants from the p-type cladding layer to the n-type cladding layer, and have found that when Ge is employed as a dopant in the p-type cladding layer, dopant diffusion is suppressed and shift of a pn junction is prevented. However, when an electrode is formed on a Ge-doped p-type cladding layer, electric resistance between the electrode and the p-type cladding layer and VF of an LED increase. The inventors have conducted further studies in order to solve the above problem, and found that when a Zn-doped p-type layer is interposed between a p-type cladding layer and the electrode, there is produced an LED which exhibits lowered electric resistance between the electrode and the p-type layer and high emitted-light intensity and has an excellent service life.
The inventors have further carried out studies on optimization of emitted-light intensity and VF in the aforementioned LED, and have found that when the thickness of the n-type cladding layer 5 is controlled to 60-80 xcexcm, intensity of emitted light of an LED is enhanced, and that when the carrier concentration of each of the n-type cladding layer and the second n-type layer is elevated, VF is lowered but intensity of emitted light is lowered. In addition, in order to attain VF of 2 V or less at 500 mA, which is a standard VF value of an LED of dimensions of 350 xcexcmxc3x97350 xcexcm, the carrier concentration of each of the n-type cladding layer and the second n-type layer must be controlled to 8xc3x971017 cmxe2x88x923 or more, whereas when the carrier concentration is in excess of 2xc3x971018 cmxe2x88x923, intensity of emitted light of an LED decreases considerably.
In the LED according to the present invention, the second n-type layer is formed on the n-type cladding layer such that the two layers are brought into mutual contact, and an n-n junction interface is formed between the two layers. When the n-type epitaxial layers including the n-n interface are inverted to the p-type layers, a thyristor effect generates to thereby disadvantageously shut off electric current. It has been found that the following two mechanisms are considered to cause conduction-type inversion from n to p.
In a first mechanism, Zn evaporated from a melt for forming the first p-type layer is incorporated into a melt for forming the n-type cladding layer or into that for forming the second n-type layer, or Ge-containing crust adhering on substrate-holders during growth of the p-cladding layer and the active layer is incorporated into a melt for forming the n-type cladding layer, to thereby elevate Zn concentration and Ge concentration of the n-type cladding layer or the second n-type layer. In a second mechanism, n-type carrier concentration of an initially grown portion of the second n-type layer decreases, since segregation coefficient of Te decreases with increasing temperature. It has been found that when Zn concentration or Ge concentration is greater than Te concentration, n-p inversion occurs in epitaxial layers involving the aforementioned n-n interface.
In addition, the LED of the present invention has a DDH structure in which a fourth layer is formed on the n-cladding layer side. Thus, the total thickness of the n-type cladding layer and the aforementioned second n-type layer accounts for most of the thickness of the LED. In order to suppress absorption of emitted light by the two layers, the inventors have conducted further studies, and have found that VF increases when the Al compositional proportion in each layer is highest; i.e., when the Al compositional proportion at initiation of growth thereof is in excess of 0.35. As has conventionally been known, in a GaAlAs mixed crystal system, when the Al compositional proportion increases, electron mobility decreases abruptly. According to J. Inst. Electron. Telecommun., Eng., vol. 29, p. 97, 1985 (A. K. Saxena), electron mobility is the lowest at an Al compositional proportion of 0.45. In the LED of the structure according to the present invention, the highest Al compositional proportion of each of the aforementioned n-type cladding layer and the second n-type layer is found to be 0.35, which is lower than the Al compositional proportion reported in the literature.
Furthermore, dopants in the p-type cladding layer and the p-type active layer have been investigated, and the inventors have found that when Ge is employed as a dopant in the p-type cladding layer and the p-type active layer, intensity of emitted light and response speed of an LED are affected by variation of carrier concentration, to thereby provide disadvantageously poor LED characteristics. The inventors have investigated further, and have found that variation in thickness of the active layer and variation in oxygen concentration of the active layer also inhibit stabilization of LED characteristics. The present invention has been accomplished on the basis of these findings.
Accordingly, the present invention is directed to the following:
[1] an epitaxial wafer for fabricating an infrared light-emitting device, which wafer is obtained by sequentially stacking, on a p-type GaAs single-crystal substrate, a first p-type layer (Ga1xe2x88x92X1AlX1As, 0.18xe2x89xa6X1xe2x89xa60.25); a p-type cladding layer (Ga1xe2x88x92X2AlX2As, 0.15xe2x89xa6X2xe2x89xa60.23); a p-type active layer (Ga1xe2x88x92X3AlX3As, 0xe2x89xa6X3xe2x89xa60.03) which has an emission wavelength of 850-900 nm; an n-type cladding layer (Ga1xe2x88x92X4AlX4As, X3 less than X4xe2x89xa60.45); and a second n-type layer (Ga1xe2x88x92X5AlX5As, X3xe2x89xa6X5xe2x89xa60.45), through liquid phase epitaxy, and, subsequently, removing the p-type GaAs single-crystal substrate, wherein zinc is employed as a dopant in the first p-type layer; germanium is employed as a dopant in the p-type cladding layer and the active layer; and the carrier concentration of each of the p-type cladding layer and the active layer is not less than 8xc3x971017 cmxe2x88x923 and less than 1.2xc3x971018 cmxe2x88x923;
[2] an epitaxial wafer for fabricating an infrared light-emitting device as described in [1], wherein each of the n-type cladding layer and the second n-type layer has an average carrier concentration of 8xc3x971017 cmxe2x88x923 to 2xc3x971018 cmxe2x88x923 and the n-type cladding layer has a thickness of 60-80 xcexcm;
[3] an epitaxial wafer for fabricating an infrared light-emitting device as described in [1] and/or [2], wherein the second n-type layer has a total concentration of Zn and Ge of 1xc3x971017 atoms/cm3 or less; Te is employed as a dopant in the n-type cladding layer and the second n-type layer; and the dopant concentration of the portion in the second n-type layer within 3 xcexcm of the interface between the n-type cladding layer and the second n-type layer is 3xc3x971017 cmxe2x88x923 or more;
[4] an epitaxial wafer for fabricating an infrared light-emitting device as described in any one of [1] to [3], wherein compositional proportions of Al in the n-type cladding layer and the second n-type layer are such that 0.10 less than X4 less than 0.35 and 0.10 less than X5 less than 0.35.
[5] an epitaxial wafer for fabricating an infrared light-emitting device as described in any one of [1] to [4], wherein the p-type active layer has a thickness of 0.5-1.2 xcexcm and an oxygen concentration of less than 3xc3x971016 atoms/cm3;
[6] a light-emitting device fabricated by use of an epitaxial wafer for fabricating an infrared light-emitting device as recited in any one of [1] to [5]; and
[7] an optical communications and spatial transmission apparatus employing a light-emitting device as recited in [6].